1. Field of the Invention
This invention relates to resonant multi-mode radiator elements for panel-form loudspeakers. In particular, resonant multi-mode radiator elements which can improve the sound quality of such speakers by reducing the occurrence of air resonance within the functional bandwidth of the speaker are considered.
2. Discussion of the Prior Art
U.S. Pat. No. 3,247,925 discloses a low frequency panel-form loudspeaker which operates by exciting bending waves in a lightweight, stiff panel which remains essentially stationary but for the bending waves. Such radiating panels may comprise skinned composites with honeycomb or similar shaped cellular cores.
Acoustic distortion may occur in panel form loudspeakers where air resonance occurs in the core of the panel. Patent application No. PCT/GB91/01262 discloses how this form of acoustic distortion can be reduced by ensuring that the frequency at which the first air resonance occurs within the core of the panel is above the functional upper frequency limit of the loudspeaker. This frequent .function..sub.core is a function of the speed of sound in air, c and the depth of the panel, d as follows; EQU .function..sub.core =c/2.multidot.d
This expression fixes the depth of the panel core according to the desired functional frequency bandwidth of the loudspeaker, the desired functional upper frequency limit, .function..sub.max, of the loudspeaker being inversely proportional to the panel depth.
It can be shown that a second level of acoustic distortion occurs in panel-form loudspeakers at certain frequencies within the panel, when a localised resonance occurs in the skin immediately above individual core cells. The effect of this localised skin resonance is to reduce the effective bending stiffness of the panel speaker and consequently the radiation efficiency of the panel.